What matters is total emissions over the modern time period from the recent past until several generations (at least) into the future. There has to be a limit, that limit has to be divided among nations, and that division has to take population into account at some level. The US can't be expected to produce the same total emissions as Mexico, and China can't be expected to produce the same as the US.

So if we set a per-capita limit, with China and India agreeing that for all of the 20th Century and for much of the 21st Century that the US can produce more per-capita, then the total per-capita emissions over the two centuries (or even just the 21st Century) will be far greater for the US. China and India are saying they're willing to accept that outcome.

China and India could easily have taken a different stance, and said that because the US produces more emissions per-capita in the first 50 years of the 21st Century (let alone the previous century), then they should be allowed to produce more per-capita in the second half. Instead they are planning to do more than we are doing.

For Republicans like Marco Rubio to say the rest of the world is doing nothing and therefore we shouldn't either, when China and India have actually committed to do more than the US by having total lower emissions, is outrageous. And no matter how you slice it, Rubio is lying about India repeatedly - even with three times the US population it is not "polluting in the atmosphere much greater than we are" - their total emissions are one-third of ours. Rubio is lying, and the rest of them are deceptive.